1. Field of the Invention
This invention relates to a bicycle hub unit, more particularly to a bicycle hub unit with bearing-receivers for mounting a cylinder drive in a hub shell thereof.
2. Description of the Relative Art
Referring to FIGS. 1 and 2, a conventional bicycle hub unit which is to be attached to a rear wheel of a bicycle is shown to comprise a hub shell 1 which has an axial bore 11 with a first axial end portion and a second axial end portion opposite to the first axial end portion. The hub shell 1 is supported rotatably on a hub shaft (not shown) that is mounted securely on the axle of the rear wheel of the bicycle. The hub shell 1 further has an annular series of external splines 12 which are formed at an outer end surface of the second axial end portion of the axial bore 11, and an annular shoulder 13 (see FIG. 2) which is formed on an inner peripheral surface of the hub shell 1 near the first axial end portion of the axial bore 11. A bolt 2 extends through the axial bore 11 of the hub shell 1 and is sleeved on the hub shaft. The bolt 2 has an externally threaded end portion 21 which extends out of the second axial end portion of the axial bore 11, and a head portion 22 which bears against the shoulder 13. A first bearing 3 is disposed within the first axial end portion of the axial bore 11. A cylinder drive 5 includes a head portion which is sleeved on the external splines 12 of the hub shell 1 and which has an annular series of internal splines 51 (see FIG. 2) meshing with the external splines 12 of the hub shell 1. Accordingly, when assembled, the hub shell 1 is co-rotatable with the cylinder drive 5. The cylinder drive 5 further includes an externally threaded end portion 52, two pair of ratchet pawls 53 mounted on an outer circumference of the same, and an internally threaded portion 54 connected threadedly to the externally threaded end portion 21 of the bolt 2 so as to fix the cylinder drive 5 to the hub shell 1. A sprocket carrier 6 is supported around the cylinder drive 5 and has an annular series of ratchet teeth 61 formed on an inner circumference thereof.
A waterproofing ring 4 is disposed on one end of the sprocket carrier 6 around the second axial end portion of the axial bore 11 and abuts against the hub shell 1.
The ratchet pawls 53 are spring-loaded and engage operably the ratchet teeth 61 such that the sprocket carrier 6 is co-rotatable with the cylinder drive 5 in a forward running direction for torque transmission, and is freely rotatable relative to the cylinder drive 5 in the opposite direction to enable inertial running of the bicycle. Thus, the sprocket carrier 6 can rotate relative to the cylinder drive 5 only in one direction.
A second bearing 7 is disposed within the sprocket carrier 6 and has an internally threaded portion 71 which is connected threadedly to the externally threaded end portion 52 of the cylinder drive 5.
A plurality of diametrically different sprockets 8, as shown in FIG. 3, are supported on the sprocket carrier 6 for co-rotation therewith.
Note that the connecting positions of the cylinder drive 5 to the locking bolt 2 and the second bearing 7 are located outside of the second axial end portion of the axial bore 11 of the hub shell 1. When the conventional bicycle hub unit is in use, force is applied on the sprocket carrier 6 and is then transferred to the connecting positions. Concentration of the applied force at only one side of the hub shell 1 shortens the service life of the conventional hub unit.